BERKELEY, CA — An international collaboration of radiochemists has used the PHILIPS
cyclotron at the Paul Scherrer Institute (PSI) in Bern, Switzerland, to
determine the volatility of bohrium, element 107 -- the heaviest element
yet whose chemistry has been successfully investigated.

Crucial to the research was the use of an isotope of bohrium with a
relatively long half-life of about 15 seconds, detected earlier this year
by researchers at the U.S. Department of Energy's Lawrence Berkeley
National Laboratory and the University of California at Berkeley. The team
worked at Berkeley Lab's 88-Inch Cyclotron with visitors from the PSI and
the University of Bern.

Although several elements heavier than bohrium have been
identified, the correct placement of the heaviest elements
in the periodic table is under study.

"In the discovery experiments of new elements, only the existence
of a new, very heavy atomic nucleus is demonstrated," says Heinz
Gäggeler, leader of the PSI team, "but no information about its
chemical properties is obtained." To date, the heaviest element whose
chemical properties have been widely investigated by experiment is
seaborgium, element 106. "Thus, in the view of a chemist,"
Gäggeler says, "the periodic table currently ends at seaborgium."

"Elements beyond 100 are made an atom at a time, with very low
production rates, and have very short half lives," says Darleane C.
Hoffman, a longtime collaborator with Gäggeler's team and coleader of the
group which identified the relatively long-lived isotope, bohrium 267, at
the 88-Inch Cyclotron. A member of the Nuclear Science Division at
Berkeley Lab, she is a professor of chemistry at UC Berkeley. Hoffman
says, "The chemistry of the heavy elements requires separations that
come to equilibrium very rapidly, and these must be valid on an
atom-by-atom basis."

Such atoms are created in the laboratory by bombarding heavy target
nuclei with an accelerated beam of projectile ions. The nuclei of
interest, which are created by the evaporation of a few neutrons, are only
a very small fraction of the huge number of reaction products produced. At
PSI, the PHILIPS cyclotron yielded about three atoms of bohrium during a
day of beam time, but only four bohrium nuclei were detected in the first
two weeks of the volatility experiment.

The PSI researchers used a beam of neon 22 to bombard a target of
berkelium 249, which has a half-life of 320 days. The targets were
prepared at Berkeley Lab from material furnished by the Department of
Energy through its Transplutonium Element Production Program at Oak Ridge
National Laboratory.

Immediately after bombardment, the reaction products were swept into an
automated isothermal system called the On-Line Automated Gas Analyzer
(OLGA) developed by Gäggeler and his colleagues. There, reaction products
formed molecules in oxygen-containing hydrogen chloride gas. These
oxychlorides were then passed through a chromatography column, in which
the more volatile species pass through at lower temperatures. In this
system, bohrium 267 compound was shown to be volatile at 180 degrees
Celsius.

The four bohrium atoms were found only after they had passed through
the chromatography column, when the oxychloride molecules containing them
were deposited on a rotating detector that carried each small sample under
a set of radiation detectors. Bohrium 267 was unambiguously identified by
the pattern of its alpha decay, first to dubnium 263, then to lawrencium
259, and subsequently to mendelevium 255.

Because the positive charge of a heavy nucleus is so great, the
electronic structure of the atom is distorted. These so-called
"relativistic effects" can produce unexpected deviations from
chemical properties extrapolated from the element's lighter homologues in
the periodic table.

Bohrium may also prove to deviate in this way. The oxychloride of
bohrium was shown to be volatile at 180 C, similar to its lighter
homologues in group VII of the periodic table, such as rhenium and
technetium. Continuing experiments will determine whether bohrium is also
volatile at lower temperatures. Technetium, for example, is volatile at 50
C and rhenium at 75 C under the same conditions.

The need to develop techniques for understanding the chemistry of the
heaviest elements is partly driven by the search for the "island of
stability," a group of superheavy elements whose nuclear shell
structure is predicted to make them stable for hundreds or thousands of
years or longer, instead of for mere seconds or milliseconds. Isotopes
with the number of neutrons required to reach the island of stability have
not yet been created.

Meanwhile, however, there is a region of relative stability due to
"deformed shells" at lower neutron and proton numbers, which
includes bohrium 267. Thus chemical studies of bohrium are not only
intrinsically interesting, but aid in what Darleane Hoffman calls
"the long march up the periodic table toward the island of
stability."

Besides Hoffman, the collaborating team at Berkeley included Berkeley
Lab senior scientists Kenneth Gregorich and Heino Nitsche, who is also a
professor of chemistry at UC Berkeley, postdoctoral fellows Uwe Kirbach
and Carola Laue, and graduate students Joshua Patin, Dan Strellis, and
Philip Wilk.

In addition to PSI, Berkeley Lab, and UC Berkeley, collaborating institutions
included the University of Bern in Switzerland, the Flerov Laboratory in
Russia, the Forschungzentrum Rossendorf, Gesellschaft für
Schwerionenforschung (GSI), and Technical University of Dresden in
Germany, and the Japan Atomic Energy Research Institute in Japan.

The Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified scientific
research and is managed by the University of California.